1. Field of the Invention
This invention relates to a method and apparatus for sealing between a tubular member and a wellhead structure from which the tubular member is suspended, and more particularly to an apparatus and method for sealing between the upper end of a well casing stub having a generally rough unfinished outer surface and a tubing head or a Christmas Tree adapter, for example.
2. Background of the Invention
Casing strings are normally suspended from a wellhead assembly. The wellhead assembly includes a casing head, which with a casing hanger, supports a casing string which extends into the well. A casing string may extend upwardly from the casing head higher than a preferred height for installation of a tubing head about the casing and with connection to the casing head. The upper extension of the casing is called a "casing stub". Normal installation includes providing a metal-to-metal seal between the exterior casing stub wall and an interior surface of the tubing head. Normal installation is accomplished with a finished exterior of the casing stub so that sealing with a metal-to-metal seal is effective. A seal is necessary between the OD of the casing stub and the ID of the tubing head to prevent possible hydrocarbon spillover from the producing strings inside the casing to the annulus outside of the casing. The tubing head which fits about the casing stub supports the production tubing which extends into the casing to a hydrocarbon pay zone in the well.
During completion operations it may be desirable or necessary under various conditions, such as when a casing string is stuck downhole, to cut off or remove an upper longitudinal section of the casing string which projects from the upper end of the casing head. When an upper section of the casing is cut off, a projecting portion of the casing remains which projects above the casing head. It is not finished. Its outer surface may be rough in texture with a Root Mean Square (RMS) roughness of about 64 RMS, but the roughness may be as high as 250 RMS. Relatively hard metal seals are used for sealing between the finished or relatively smooth OD of the projecting upper end section of a casing and the adjacent ID sealing surface of a tubing head. A hard metal seal may not be acceptable in sealing about unfinished or relatively rough textured casing due to serrations on the casing caused by the sharp seal teeth.
Regular length casing which has not been cut has a sealing surface which has been finished or machined with a RMS roughness less than about 32. Therefore in the past when a top end of a casing has been cut off to produce a casing stub, it has been common to machine or otherwise finish the outer surface of the casing stub to provide a smooth, finished surface for a metal-to-metal seal to be installed between the casing stub and the tubing hanger, particularly when a relatively hard metal seal is used.
Installation of a metal seal is furthermore desired to be accomplished without regard to the particular arrangement of the installed casing head, because different casing head arrangements are provided by different manufacturers.